1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) demodulator, an integrated circuit for OFDM demodulation and an OFDM demodulation method used in demodulating equipment, such as terrestrial digital broadcasting.
2. Description of the Prior Art
An OFDM system enables transmission of a large number of individual carrier waves (carriers) at a high spectral density level which are orthogonally spaced in frequency and therefore do not interfere with each other within a transmission bandwidth. The data streams are allocated in the amplitude and phase of each carrier. This transmission technique efficiently performs digital modulation which can be Phase Shift Keying (PSK), Quadrature Amplitude Modulation (QAM), etc. Also, OFDM has a characteristic of being sufficiently resistant to multipath effects and fading interference (i.e., radio waves reflected by many types of surfaces, etc.). As for establishing a standard of digital terrestrial broadcasting which employs an OFDM technique, two leading systems known as ISDB-T: Integrated Services Digital Broadcasting-Terrestrial developed in Japan and DVB-T: Digital Video Broadcasting-Terrestrial developed in Europe have been proposed.
FIG. 12 is a line block diagram of a demodulator circuit proposed for use in a conventional terrestrial digital broadcasting receiver. Referring to FIG. 12, after a signal tuned in with a tuner 1 is converted into a digital signal by an Analog/Digital (A/D) converter 2, this digital signal is then inputted into a Fast Fourier Transform (FFT) 3 to transform the signal from an OFDM symbol domain signal (time domain) to an OFDM carrier domain signal (frequency domain). The OFDM carrier domain signal is outputted from the FFT 3 is inputted into a synchronizing signal circuit 4 and a waveform multiplication circuit 5. The synchronizing signal circuit 4 extracts a synchronizing signal (Scattered Pilot (SP) signal) from an OFDM carrier domain signal outputted from the FFT 3 and then outputs to a waveform multiplication circuit 5. Also, the OFDM carrier domain signal inputted into the waveform multiplication circuit 5 is outputted through a demodulator circuit 6, an error correction circuit 7, etc. after wave form equivalence processing is performed using a synchronizing signal outputted from the synchronizing signal circuit 4.
FIG. 13 is a drawing showing how the signal correction coefficients are derived from a constructive concept view of a received data signal in a conventional terrestrial digital broadcasting receiver. The horizontal direction shows the “frequency” direction which consists of different “carriers” (carrier waves). The vertical direction shows the “time” direction which consists of “symbols” which each define one data transmission unit. Similarly, the “carriers” are lined up along the horizontal direction and “symbols” are lined up in the vertical direction. The white circles in the drawing represent a carrier for a data signal. The black circles represent a carrier called known power and a synchronizing signal (Scattered Pilot or SP signal) with phase information. The white circles with hatching are data carriers storing data, such as images and audio, with a non-inserted synchronizing signal. For example, a synchronizing signal of the digital terrestrial broadcasting (ISDB-T) in Japan is inserted at intervals at the rate of one every twelve carriers in the frequency axis direction, as well as shifts and inserts in the frequency axis direction one carrier every three symbols in the time axis direction. Furthermore,_the signal inserted in the transmission channels consists of segments whose parameters can be selected independently of each other. The ISDB-T system described above has adopted this type of Band Segmented Transmission OFDM (BST-OFDM).
Referring to FIG. 13, when a carrier signal segment like the one shown as a circle with hatching is distinguished as a carrier signal segment object for correction, a calculation method of the “signal correction coefficients” is applied. For example, a method which derives the calculated value with a straight line filter based on a plurality of synchronizing signals within the same symbol. Specifically, the method of predicting these coefficients uses a plurality of synchronizing signals S (n, m+2) and S (n+12, m+2) within the same symbol of the carrier signal segment object for correction S (n+6, m+2). Another method of predicting is based on the synchronizing signals between a plurality of symbols. Specifically, the prediction method using the synchronizing signals S (n+6, m) and S (n+6, m+4) over a plurality of symbols.
However, the prediction method based on a plurality of synchronizing signals within the same symbol is difficult to accomplish. Particularly, when a carrier signal segment object for correction is positioned close to the synchronizing signals, the proper adjustment can be performed. However, when a carrier signal segment object for correction is not positioned near the synchronizing signals, the proper adjustment is not easily established as there is a drawback of generating phasing errors.
The prediction method based on synchronizing signals between a plurality of symbols is an effective method only when there are no timing changes. When there are timing changes, proper adjustment is problematic and contains the same fault of being susceptible to generating an unacceptable error rate.
In addition, since the above-mentioned conventional example performs adjustment of only the proximity synchronizing signals, when a rapid change in the receiving environment occurs and the synchronizing signals received in either S (n, m+2) and S (n+12, m+2) are degraded or unrecognizable, the signal correction coefficients are also received with large variations. Thus, there is a drawback that demodulation processing of erroneous data will transpire. In other words, since channel variations during even one symbol will cause carrier interference in OFDM systems, these timing variations directly impact overall system performance by the loss of the carrier orthogonality.
As described in the above-mentioned conventional example, the overall performance of demodulating equipment deteriorates substantially under a multipath environment which is a composite waveform of two or more waves. Furthermore, as a level of Doppler phenomenon is created when a receiver shifts, the Carrier to Noise (C/N) ratio becomes higher and when significant becomes impossible to receive a signal below its C/N value.